In the formation of ports, or bores within metal workpieces, it is often necessary to machine a port having a complex shape. For example, in the formation of ports for hydraulic fluid line couplings, the port needs to provide a conduit for the hydraulic fluid, an area, often threaded, for the fastening of the coupling to the metal workpiece, an annular rim or countersink for receiving an O-ring or similar sealing mechanism, and a spot face or top annular surface which flushly butts up against the hydraulic coupling.
Given the complex shape of such ports, either a number of different tools must be employed to form the ports, or a tool capable of making multiple cuts must be used. If a number of separate tools are used in the formation of the port, the fabrication process is necessarily slowed. Therefore, it is advantageous to combine multiple cutting functions into a single cutting tool.
As a result, such bores are typically formed by a tool performing multiple functions in the form of a cylindrical body which is rotated by a torque producing machine and which is provided with cutting surfaces at various cutting levels or stations along the cylindrical body. The cutting surfaces can either be machined directly into the cylindrical body, or can be provided in the form of indexable cutting inserts. A pilot hole is initially formed in the metal workpiece, and then the porting tool rotatingly penetrates the pilot hole, with the smallest diameter cutting surface enlarging or boring the diameter of the pilot hole, and the other, larger diameter, cutting surfaces providing annular channels, countersinks or spot faces in the port for the aforementioned reasons.
Such prior art devices are disclosed in Benhase, U.S. Pat. Nos. 4,505,626 and 4,531,867 which use cylindrical bodies having three distinct cutting inserts attached thereto. One cutting insert is provided at the smallest diameter portion of the body to bore the pilot hole and enlarge its diameter, the second cutting insert is provided in a next larger diameter section of the cylindrical body to provide a countersink in the bore, and a third cutting insert is provided at the largest diameter portion of the cylindrical body to provide a spot face after the bore has been bored and countersunk. In addition to the fact that such a device requires three separate cutting inserts, its axial feed rate, or the rate at which it can produce bores, is limited in that it takes a full revolution of the tool to make a full circumferential cut through the bore.
In addition to the relatively limited axial feed rate of prior art devices, the use of cutting inserts that are radially disposed about the circumference of the cylindrical body, and which do not extend across the entire diameter of the shaft, results in chatter or deflection of the shaft as it advances through a workpiece. This phenomenon is due to the fact that as the tool engages the workpiece, the cutting insert is confronted with radial forces tending to push the shaft out of perfect rotational movement about its longitudinal axis. Since the ports formed by these tools are often used to receive hydraulic couplings conveying pressurized hydraulic fluid, the dimensions of the bore must be precise to allow for a complete and accurate seal between the coupling and the metal workpiece as well as a complete compression of the O-ring or other sealing mechanism. With the prior art devices mentioned above, the deflection of the rotating tool often results in undersized bores, and the chatter often results in ridged or undulating surfaces in the bore, both of which detrimentally affect the attainment of these objectives.